Along with recent widespread use of portable electronic devices such as mobile phones and laptop personal computers, there has been a strong demand for development of small, lightweight nonaqueous electrolyte secondary batteries having high energy density. There also has been a strong demand for development of high-output secondary batteries for use in electric vehicles including hybrid vehicles. Batteries in electric vehicles, in particular, are used in a wide temperature range from high temperatures to extremely low temperatures and therefore are required to be high output in this wide temperature range.
Such high-output secondary batteries include nonaqueous electrolyte secondary batteries such as lithium-ion secondary batteries. A lithium-ion secondary battery is essentially constituted of a negative electrode, a positive electrode, and an electrolyte solution. Active materials used in the negative electrode and in the positive electrode are each a material capable of deintercalating and intercalating lithium ions.
Such nonaqueous electrolyte secondary batteries are being actively researched and developed. Among these, ones including a layered or spinel lithium-metal composite oxide as the positive electrode material have high voltage, as high as 4 V, and are therefore increasingly used in practical settings where high-energy-density batteries are required.
As a positive electrode material, there have been proposed lithium-metal composite oxides, such as lithium-cobalt composite oxide (LiCoO2), which is relatively easily synthesized, lithium-nickel composite oxide (LiNiO2) containing nickel that is less expensive than cobalt, lithium-nickel-cobalt-manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), lithium-manganese composite oxide (LiMn2O4) containing manganese, and lithium-nickel-manganese composite oxide (LiNi0.5Mn0.5O2).
Among these positive electrode materials, lithium-nickel-cobalt-manganese composite oxide that has excellent thermal stability and high capacity has been receiving attention in recent years. The lithium-nickel-cobalt-manganese composite oxide is a layered compound, just like lithium-cobalt composite oxide and lithium-nickel composite oxide are, and contains nickel, cobalt, and manganese substantially at a ratio of 1:1:1 at its transition metal sites.
Because of its low cobalt ratio compared to that of lithium-cobalt composite oxide (LiCoO2), however, the lithium-nickel-cobalt-manganese composite oxide (LiNi1/3Co1/3Mn1/3O2) when used as a positive electrode material tends to result in poor output characteristics, high resistance, and a reduced likelihood of achieving a high-output outcome.
In the circumstances, a positive electrode material that achieves excellent battery performance (excellent cycle characteristics, high capacity, and high output) is sought after, and a technique is proposed that includes addition of tungsten or another metal to a lithium-metal composite oxide.
Patent Literature 1 proposes lithium cobalt oxide or lithium nickel oxide to which at least one element selected from B, Bi, Mo, P, Cr, V, and W is added. According to the inventors, the presence of the added element allows active movement of substances in the liquid phase, facilitates particle growth, facilitates formation of particles that have a smooth and even surface, reduces the specific surface area of LiCoO or the like to be synthesized, allows efficient action of the conductive aid acetylene black that is added to the positive electrode active material during battery fabrication, enhances electronic conductivity of the positive electrode material, and significantly lowers the internal resistance of the resulting battery.
Patent Literature 2 proposes a positive electrode active material for nonaqueous electrolyte secondary batteries, the positive electrode active material including at least a composite oxide that is composed of lithium and transition metal and has a layered structure, the composite oxide of lithium and transition metal being in a form of particles consisting of either primary particles or agglomerated primary particles, namely, secondary particles, or both of these particles, the aspect ratio of the primary particles being from 1 to 1.8. At least on the surface of the particles of the composite oxide of lithium and transition metal, a compound is present that contains at least one kind selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine. According to the inventors, the presence of the compound containing at least one kind selected from molybdenum, vanadium, tungsten, boron, and fluorine on the particle surfaces enhances conductivity.
Patent Literature 3 proposes powders of a compound of lithium and transition metal, for use as a positive electrode material for lithium secondary batteries. The powders are mainly composed of a compound of lithium and transition metal, the compound having a function to intercalate and deintercalate lithium ions. The powders are formed by adding, to this main component, a single kind of compound containing at least one element selected from B and Bi and a single kind of compound containing at least one element selected from Mo, W, Kb, Ta, and Re, followed by firing. According to the inventors, the firing process after addition of the added compounds causes formation of fine powders of the compound of lithium and transition metal while suppressing particle growth and sintering and can therefore cause formation of powders of a lithium-containing transition metal compound that are improved in the rate, improved in the output characteristics, easy to handle, and easy to be prepared into an electrode.
Patent Literature 4 proposes a positive electrode composition for nonaqueous electrolyte solution secondary batteries, the positive electrode composition including a composite oxide of lithium and transition metal of the general formula LiaNi1-x-yCoxM1yWzM2wO2 (where 1.0≤a≤1.5, 0≤x≤0.5, 0≤y≤0.5, 0.002≤z≤0.03, 0≤w≤0.02, 0≤x+y≤0.7, M1 is at least one kind selected from the group consisting of Mn and Al, and M2 is at least one kind selected from the group consisting of Zr, Ti, Mg, Ta, Nb, and Mo) as well as a boron compound that contains at least the element boron and the element oxygen. The inventors claim as follows: because the positive electrode composition includes not only the composite oxide of lithium and transition metal essentially containing nickel and tungsten but also a particular boron compound, the output characteristics and the cycle characteristics are enhanced compared to the case where the positive electrode composition solely includes the composite oxide of lithium and transition metal.
Another technique is proposed that uses a positive electrode including particles having a uniform and appropriate size and a hollow structure and therefore achieves excellent battery performance (excellent cycle characteristics, low resistance, and high output).
Patent Literature 5 proposes a positive electrode active material for nonaqueous electrolyte secondary batteries including lithium-nickel-manganese composite oxide, the lithium-nickel-manganese composite oxide being composed of lithium-containing composite oxide that has a layered hexagonal crystal structure and being represented by the general formula Li1+uNixMnyCozMtO2 (where −0.05≤u≤0.50, x+y+z+t=1, 0.3≤x≤0.7, 0.1≤y≤0.55, 0≤z≤0.4, 0≤t≤0.1, and M is one or more added elements that are selected from among Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, and W), the positive electrode active material having an average particle size of 2 μm to 8 μm, having a value [(d90-d10)/average particle size], which is an index of the extent of particle size distribution, of 0.60 or lower, and having a hollow structure that has an outer shell section composed of agglomerated sintered primary particles and a hollow section present inside the outer shell section. According to the inventors, the use of this positive electrode active material can achieve high capacity, excellent cycle characteristics, and high output in the resulting nonaqueous secondary battery.